I thought the EU had regulations on credit card fees? As I understand it, the Amex isn't widely supported because Amex's fee-heavy business model wouldn't be allowed to launch.

Visa's highest tier cards (Visa Infinite cards, such as the Chase Sapphire Reserve), already have higher fees than the Amex Platinum. The difference is, Visa won't let merchants ban single cards (you have to accept ALL Visa cards), while banning Amex meaning you are banning mostly high tier cards and losing nothing on the low end.